Pasterurella multocida is classified into family Pasteurellaceae, genus Pasteurella, and multocida means “causing many kinds of animal disease” in Latin. In the past, the taxonomic situation of Pasteurellaceae was ambiguous and arguable. However, according to the sequence analysis of 16s-RNA, Pasteurellaceae is classified into a gamma subgroup of purple bacteria, which is Gram-negative facultative anaerobe, and homologous to Enterobacteriaaceae. Pasteurella multocida is an important pathogen of domestic animals and an opportunistic pathogen of humans. Human infections with P. multocida largely arise from the bite of an infected carnivore, but other types of infections are occasionally reported (Hubbert W T, et al., Am J Public Health 1970; 60:1109-17). P. multocida has wide host range, and is the causative agent of fowl cholera in domestic birds, haemorrhagic septicaemia in cattle or sheep, and atrophic rhinitis in pigs (Hunt M L, et al., Vet Microbiol 2000, 72:3-25). Most P. multocida strains are highly pathogenic to murine and rabbits, which can result in acute septic symptoms at an infection dose of 1-10 CFU.
In domestic birds, P. multocida causes acute septic disorders in turkey, chicken, duck, and goose, which may lead to a great loss in economy. Of the five capsular serotypes (A, B, D, E and F) and 16 LPS serotypes, fowl cholera is mainly caused by serotypes A:1, A:3 and A:4 (Glisson J R. In: Saif Y M, editor. Diseases of poultry. Iowa State University Press, Ames, Iowa, 2003:657-90). Although both live-attenuated vaccines and bacterins are available, outbreaks of fowl cholera continue to occur. Live-attenuated vaccines have the disadvantage of reversion to virulence, while bacterins do not protect hosts against heterologous challenge (Bierer B W, Derieux W T, Poult Sci 1972; 51:408-16; and Rebers P A, Heddleston K L, Avian Dis 1977; 21:50-56). These disadvantages call for the development of a new type of vaccine for P. multocida. 
In a previous report, a lipoprotein, designated Pasteurella lipoprotein E (PlpE), from Mannheimia haemolytica (formerly known as Pasteurella haemolytica), was found to be highly immunogenic in cattle (Confer A W, et al., Vaccine 2003; 21:2821-9). PlpE is a lipid-modified, surface-exposed outer membrane protein that is important in complement-mediated killing of M. haemolytica (Pandher K, et al., Infect Immun 1998; 66:5613-9). Addition of recombinant PlpE to the commercial M. haemolytica vaccine markedly enhanced the vaccine-induced resistance against experimental challenge with serotypes 1 and 6. A bioinformatics-based sequence search showed that a gene annotated PlpE is present in the published genome sequence of P. multocida strain pm-70 (serotype A:3) (May B J, et al., Proc Natl Acad Sci USA 2001; 98:3460-5). This gene has the potential to encode a lipoprotein of 335 amino acids that has 24.3% sequence identity with PlpE of M. haemolytica and 19.1% identity with OmlA of A. pleuropneumoniae. It has not yet been determined whether the PlpE of P. multocida could serve as a vaccine antigen by using mice and/or chicken as animal models.
Therefore, the present invention first finds out a new use of Pasteurella lipoprotein E (PlpE) in controlling infective diseases caused by P. multocida, or related disorders thereof, and further develops a subunit vaccine for protecting animal from diseases caused by P. multocida. The subunit vaccine of present invention is characterized by using recombinant PlpE protein as active antigen, which has no adverse side effect of forming fibrosarcoma. Additionally, the present subunit vaccine of P. multocida can provide a predominant protective effect against the challenge of homologous and/or heterologous serotypes in immunized animals over traditional inactivated or live-attenuated vaccines.